This is a known issues of OTP 21 on windows:
- Having 2 issues with Elixir mix task after upgrading to Elixir 1.6.6 and Erlang 21
- https://bugs.erlang.org/browse/ERL-644
- https://github.com/elixir-lang/elixir/issues/7774
You need to downgrade your erlang installation.
